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Mayumi Kitagawa, Jun-Ichi Murata, Haruo Uesugi, Ritsuko Hanajima, Yoshikazu Ugawa and Hisatoshi Saito

Object

The aim of the present study is to evaluate the topographical distribution of somatosensory evoked potentials (SSEPs) in the subthalamic area, including the zona incerta (ZI). Determination of this distribution may help in the correct placement of deep brain stimulation (DBS) leads.

Methods

Intraoperative SSEPs were recorded from contacts of DBS electrodes at 221 sites in 41 patients: three patients with essential tremor and 38 with Parkinson disease who underwent implantation of DBS electrodes for the relief of severe tremor or parkinsonism.

Results

Two distinct SSEPs were recorded in the subthalamic area. One was a monophasic positive wave with a mean latency of 15.8 ± 0.9 msec, which the authors designated subthalamic P16. Using both cephalic and noncephalic references, subthalamic P16 was only recorded in the ventral part of the ZI (mean 6.6 ± 1. 3 mm posterior to the midcommissure point, 4.8 ± 1.2 mm inferior to the anterior commissure–posterior commissure line, and 9.7 ± 0.6 mm lateral to the midline). When bipolar recordings were made, the traces showed a phase reversal at the caudal part of the ZI. The second potential is a positive–negative SSEP recorded throughout the entire subthalamic area. The mean latencies of the initial positive peak and the major negative peak were 13.6 ± 1.1 msec and 16.4 ± 1.1 msec, respectively. Several small notches were superimposed on the peaks, and their amplitudes were largest at the contact close to the medial lemniscus.

Conclusions

The results indicate that intraoperative SSEPs from DBS electrodes are helpful in refining stereotactic targets in the thalamus and subthalamic areas.

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Jun-ichi Murata, Mayumi Kitagawa, Haruo Uesugi, Hisatoshi Saito, Yoshinobu Iwasaki, Seiji Kikuchi, Kunio Tashiro and Yutaka Sawamura

Object. Tremors, including its proximal component, are often refractory to standard thalamic surgery. In the 1960s the posterior part of the subthalamic white matter was reported to be a promising target in treating various forms of tremor, but was also found to be associated with adverse effects. Advances involving a less invasive method, that is, deep brain stimulation (DBS), has led to a reappraisal of this target.

Methods. Eight patients with severe essential tremor involving the proximal arm were treated using unilateral stimulation of the posterior part of the subthalamic white matter. The tentative target was situated in the area lateral to the red nucleus and posteromedial to the subthalamic nucleus. Macrostimulation was used to find the optimal site to suppress tremor. Through a quadripolar DBS lead, somatosensory evoked potentials (SSEPs) were recorded. Improvement of tremor was evaluated based on a modified clinical tremor rating scale. Anatomical locations of all contacts were assessed using stereotactic guidance and represented on the Schaltenbrand—Wahren atlas.

Conclusions. A characteristic diphasic pattern of SSEPs reaffirmed the electrophysiological endorsement of this target. Tremors, both proximal and distal, were remarkably improved in all patients. The rate of improvement, as indicated by the total tremor score, was a mean of 81%. Axial tremors in the legs and head were also improved. Most of the contacts associated with remarkable improvement were located in the posterior part of the subthalamic white matter (the zona incerta and prelemniscal radiation). Neither major complications nor neurological deterioration was observed. The authors concluded that DBS of the posterior part of the subthalamic white matter together with SSEP recording is a safe and effective method to ameliorate severe intractable tremors.